I watched a video of "Glass cutting" which uses a Water Jet Cutter. It was said that Cutting glass simply by machines would eventually crack it... So, They're using grains of sand (by placing a sand paper beneath) to cut glass using a Water Jet. It also has enough force to cut through steel.

What exactly is the property of GLASS so that it requires a "Water Jet" to cut it..?

Also, Why can't we use a powerful laser and some kinda coolant to cut through glass..?

3 Answers
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Cutting glass with a water jet uses grains of hard material (such as sand but occasioanlly harder minerals or diamond) to grind away at the glass one particle at a time. The water jet is just used to carry the sand to the glass at high speed and remove it and the eroded glass. As the poster says, it also cools the glass and so prevent cracking.

Cutting glass with a laser is harder. Laser cutting in most materials involves heating a region until it melts or evaporates - unless you have a material where there is a very strong absorption for the laser wavelength and you can use very short pulses to break bonds efficently.
So in glass or metal this means a lot of energy is absorbed by the surrounding material - which will heat up and expand - and in the case of glass, crack.

To complete Martin's answer: the result of Laser cutting is non smooth because of the melted leftover on both side of the cut. You can experience it with the nylon sticker on clothes. They are laser cut and their rim is hardened up to the itching point...